Emergency departments (ED) are overburdened with patients with serious mental illness (SMI) experiencing acute mental health crises. Since 2007, the number of ED visits has increased from 95 million to over 140 million annually and the percentage of ED visits related to a primary mental health disorder has increased by 18%, outpacing the growth among all other ED diagnoses.1 Furthermore, the ED-based care of patients with SMI in crisis is inefficient, costly, and of poor quality. The average ED length of stay for a patient in North Carolina with SMI is 62 hours (over 15 times longer than the typical ED patient) and characterized by excessive use of restraints, disproportionate sedating anti-psychotic injections, high hospitalization rates, and runaway costs.2,3 There is an urgent need to identify and test alternative crisis intervention strategies that better match SMI patient needs with care resources and that reduce the burden on EDs. Prehospital emergency medical services (EMS) is a promising mechanism to reduce ED use and improve patient care by rapidly and safely diverting appropriate SMI patients in crisis to alternative treatment settings.4 Recently, EMS in Wake County, NC (population 1.3 million) implemented a large, pilot intervention to divert 9-1-1 patients experiencing a behavioral health crisis to a dedicated crisis and assessment services unit located within a nearby community mental health center (WakeBrook) instead of the ED.5,6 Since 2013, this novel, pilot EMS program has evaluated over 1,000 patients in crisis, 50% of whom were diverted away from the ED. Other communities have begun experimenting with similar programs, although the use of EMS- diversion has far outpaced our scientific understanding of its safety, acceptability, and effectiveness. This large, innovative pilot program in Wake County provides a unique opportunity to scientifically evaluate the intervention and prepare for a future, large-scale services study of EMS intervention in acute mental health crises. This R34 application requests two years of funding to evaluate this pilot EMS intervention program for patients with acute mental health crises, including the intervention's safety, acceptability, and preliminary efficacy. These findings will significantly advance the science of EMS interventions for acute mental illness and enable us, and other investigators, to undertake future, larger-scale studies of this novel care approach. By growing the evidence-base for the integration of EMS and community-based crisis care, this research will promote the adoption and dissemination of evidence-based practices to enhance acute crisis treatment for persons with SMI. Successful broader implementation of such programs could significantly improve the care of patients with acute mental health crises and reduce ED volumes, further enhancing the broad public health significance of our research agenda. 1